1. Field of the Invention
This invention relates to hydraulic turbine installations. More particularly, this invention relates to an adjustable draft tube for a hydraulic turbine installation.
2. Description of the Prior Art
Hydraulic turbine draft tubes provide a gradually expanding discharge passage for water flowing from a turbine runner to a tailwater. The draft tube supplements the action of the turbine runner by using most of the energy remaining in the water discharging from the runner. Draft tubes are shaped to decelerate water flow with a minimum of loss so the kinetic energy of the flow discharged from the runner may be efficiently regained by conversion into a suction head thereby increasing the total pressure difference on the runner. The draft tube also serves to regain static suction head in cases where the runner is located above the tailwater level.
For a draft tube to operate properly, its discharge opening must be adequately submerged beneath the tailwater level. If the discharge opening is inadequately submerged, air can enter the draft tube and the suction head could be lost.
The tailwater level for a hydraulic turbine installation may vary considerably due to seasonal flooding and other influences. To account for variations in tailwater level, prior art hydraulic turbine installations submerged the draft tube as much as possible. At the very least, the draft tube was positioned to be submerged below a predetermined low tailwater level. While this practice maintained the draft tube's seal by preventing air flow into the draft tube, the practice necessitated a substantial amount of excavation with a corresponding increase in cost. For economically marginal turbine installations, the high excavation costs of adequately submerging a draft tube could make the installations economically unfeasible.